The term magnetic resonance systems means magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) which in this specification will be referred to as MR systems. In such systems, a material under investigation is exposed to a static magnetic field and a radio-frequency magnetic field to generate magnetic resonance in the material, and a property of the magnetic resonance is detected which is dependent on the structure of the material and of the environment of nuclei within that material.
In a paper by C. E. Hayes et al., in the Journal of Magnetic Resonance, Volume 63, 1985, pages 622-628, an r.f. coil known as a birdcage resonator is described, comprising two circular conducting end rings connected by straight parallel conductors, known as "legs". The birdcage resonator is suitable for use in MR systems in which the static magnetic field is parallel to the magnetic bore, i.e. horizontal. The r.f. field produced by a birdcage resonator is highly homogeneous, an essential factor in MR systems, and the resonator can be used in quadrature to improve the sensitivity of the MR system, and it can also be dual-tuned to perform simultaneous measurements on two types of nuclei within the material under test.
However for horizontal magnet bore MR systems with a vertical static magnetic field, the birdcage cannot be used in quadrature. In addition, if the birdcage in the design as described by Hayes is used in an MR system with a horizontal bore, access to the sample/patient is severely limited by the legs on the resonator.
Solenoidal coils are commonly used for vertical field MR systems as whole-body and head coils. However, unlike birdcage resonators, they cannot be driven in quadrature. Further, their cylindrical structure is inherently claustrophobic, a major disadvantage for whole body MR systems. Both the solenoidal and the birdcage coil have one common disadvantage: they offer very limited side access to the patient/sample.
An alternative coil, known as a cross-ellipse coil, is described in UK Patent No. 2144918, National Research Development Corporation, which has an advantageously open structure, thus minimising claustrophobic effects, but such a coil is inherently less sensitive and less efficient than a birdcage resonator.
In DE 3347597, Philips, FIG. 15 discloses a resonator having a barrel-shape, but the precise shape of the curve is not specified, and the resonator is not a birdcage resonator.